1. Field of the Invention
This invention relates generally to TVRO receivers for the reception of a wide range of satellite TV signals. More particularly, this invention relates to improved filtering means for reducing signal distortion resulting from interference signals, such as terrestrial interference (TI) and the like, in a TVRO receiver system.
2. Description of the Related Art
In TVRO systems, satellite signals are received by earth-station antennas and down-converted to a first intermediate frequency (IF) level at the antenna location. The conversion may involve the use of a down-converter for converting only a single channel of signals from the overall broadcast signals to the 1st IF frequency. More commonly, however, a block converter is used for converting all channels of a common polarity to the first IF block of frequencies. Typically, the first IF frequency block is spread across a bandwidth of about 600 to 800 MHz. within a frequency range of 950 to 1750 MHz. frequencies is subsequently relayed to the receiver via coaxial cable so that a particular channel may be selected for reception. During the reception process, the 1st IF signals are further down-converted to a second IF frequency range which traditionally has been centered at 70 MHz. in many TVRO systems.
A significant problem in the reception of satellite broadcast TV signals using conventional TVRO receivers is the signal distortion resulting from the presence of terrestrial interference (TI) signals. TI is generated by the presence of local terrestrial microwave communication links in the vicinity of earth-station reception equipment and can cause substantial degradation and even complete loss of broadcast signals being received from a satellite.
In the past, a variety of filters have been used within TVRO receivers to perform wave trapping of TI signals within the interference region. The use of TI filters for notching out destructive TI signals is effective and convenient at the conventionally used low 2nd IF frequencies of the order of tens of MHz. However, recent generation TVRO receivers are increasingly employing a higher second IF frequency of the order of hundreds of MHz. for a variety of reasons including the realization of VCO frequencies which are substantially alienated from the 1st IF frequency range in order to prevent interference between the VCO signals and the received signals.
At the present time, for instance, the use of a 2nd IF frequency of 480.5 MHz. is common within the United States, while a 2nd IF frequency of about 480 MHz. has virtually become the standard across Europe. At these higher 2nd IF frequencies, it is difficult to design traps or notch filters having the high Q factor and temperature and aging stability essential for effective elimination of TI and other related noise signals without inducing excessive attenuation of the received signals.
The rejection of interference signals with high sensitivity and selectivity is also important in a variety of other applications. In the design of high performance frequency synthesizers, for instance, spurious signals are generated by digital signals and surface as sidebands on the desired carrier. In such applications, it is important to have a filter which has a high quality factor and provides a narrow notch for accurately rejecting the interference signals.